Medical implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, the replacement of ruptured vessels, and the treatment of disease such as vascular disease by local pharmacotherapy, i.e., delivering therapeutic drug doses to target tissues while minimizing systemic side effects. Such localized delivery of therapeutic agents has been proposed or achieved using medical implants which both support a lumen within a patient's body and place appropriate coatings containing absorbable therapeutic agents at the implant location.
The delivery of expandable stents is a specific example of a medical procedure that involves the deployment of coated implants. Expandable stents are tube-like medical devices, typically made from stainless steel, Tantalum, Platinum or Nitinol alloys, designed to be placed within the inner walls of a lumen within the body of a patient. These stents are typically maneuvered to a desired location within a lumen of the patient's body and then expanded to provide internal support for the lumen. The stents may be self-expanding or, alternatively, may require external forces to expand them, such as by inflating a balloon attached to the distal end of the stent delivery catheter.
Because of the direct contact of the stent with the inner walls of the lumen, stents have been coated with various compounds and therapeutic agents to enhance their effectiveness. These coatings may, among other things, be designed to facilitate the acceptance of the stent into its applied surroundings. Such coatings may also be designed to facilitate the delivery of one of the foregoing therapeutic agents to the target site for treating, preventing, or otherwise affecting the course of a disease or tissue or organ dysfunction.
Where the stent has been coated, care must be taken during its manufacture and delivery within the patient to ensure the coating is evenly applied and firmly adherent to the stent, and further that the coating is not damaged or completely removed from the implant during the deployment process. When the amount of coating is depleted the implant's effectiveness may be compromised and additional risks may be inured into the procedure. For example, when the coating of the implant includes a therapeutic, if some of the coating were removed during deployment, the therapeutic may no longer be able to be administered to the target site in a uniform and homogenous manner. Thus, some areas of the target site may receive high quantities of therapeutic while others may receive low quantities of therapeutic. Similarly, if the therapeutic is ripped from the implant it can reduce or slow down the blood flowing past it, thereby, increasing the threat of thrombosis or, if it becomes dislodged, the risk of embolisms. In certain circumstances, the removal and reinsertion of the stent through a second medical procedure may be required where the coatings have been damaged or are defective.
The mechanical process of applying a coating onto a stent may be accomplished in a variety of ways, including, for example, the spraying of the coating substance onto the stent and so-called spin-dipping, i.e., dipping a spinning stent into a coating solution to achieve the desired coating. Common to these processes is the need to apply the coating in a uniform manner to ensure an intact, robust coating of the desired thickness is formed on the stent. In order to achieve the desired uniform and complete coating, it has been common for the stents to be handled individually, with each stent separately loaded onto a stent holder and the coating applied to the stent before the next stent is coated. This individual handling typically has resulted in low production rates of coated stents. A further disadvantage of these prior stent coating processes is that, because the stents are wire mesh structures with substantial void area between the mesh wires, the utilization of the stent coating material sprayed toward the stents is very low. For example, in some cases the amount of stent coating sprayed toward the stent which actually adheres to the stent mesh is less than five percent.
Thus, there is a need for a method for coating stents which efficiently applies the stent coating material in a manner that results in a high quality, uniform coating on the stents at high coated stent production rates.